Try getting the 4.0.0.274 from Rogers and installing it on your Cingular. Also try installing BlackBerry Desktop Version 4.0.0.33 -- I think that may improve things.

I heard that OS ending in ".198" and Desktop ending in ".30" sometimes have problems, which are typically solved by upgrading. Cingular has not posted the newer 4.0 yet, but you can workaround this by downloading the Rogers version and deleting the VENDOR.XML file.